Field disconnect and VFD

JeffKiper

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How do you handle disconnects in the field and a VFD? I was thinking of taking out the enable with the AUX contacts on the drive. My drives are Safe-Off drives so do I kill the safe-off instead? I will monitor the drive for running and faulted. These are forward only applications.

If I am giving the drive an enable and run I should have a NOT FAULTED and a RUNNING feedback. If I don't get a RUNNING and a NOT FAULTED I will drop out the signal to RUN ,ENABLE, and SAFE-OFF.

If I kill the safe-off i think if faults the drive. Causing someone to manually reset the system. Not necessarily a bad thing if the drive is running and someone cycles the disconnect in the field. They are trying to open a disconnect under a load. NOT a good thing for a standard disconnect.

If I kill the enable the drive will coast to a stop. They can close the disconnect and restart the system no problem.

What are you thoughts
 
They make local disconnects with an early break set of contacts (EXAMPLE). I would put this in series with the drive enable. Since you are monitoring the 'running' state, you should be able to detect when the drive is disabled, and take whatever action you need.

🍻

-Eric
 
I plan on using the early break monitoring contacts. I couldn't decide what signal to break. Each one has a pro and con I have never had to.
 
If I kill the safe-off i think if faults the drive.
The problem with this method is that if they use the disconnect for its intended purpose, isolating the motor for service, it will also generate a fault. I like the enable circuit better, but it really depends on the application. Of course, the drive must be setup as 'coast to stop' when the enable is lost.

🍻

-Eric
 
If you break the safe off and then issue a run command, I think you will generate a fault something like 'Hardware Enable'. I am not sure if this fault is latched, or if it clears when the safe-off condition is returned to enabled or when the run command is removed.

In either case, you want the disconnect aux to cause a coast type of stop (drive immediately stops its output), and you don't want to try to issue run commands while the safe off is open, so both might be the best and simplest answer.

I don't see the need to use the safe off input in series with a disconnect aux. The disconnect main poles protect the machine and people, and the aux. contact to disable the drive is simply there to make life gentler for the drive, not necessarily for any safety reason...

I have even wired the disconnect aux. only as PLC inputs, then let the PLC yank the run command. This can simplify wiring, and the PLC can report the status of the disconnect as well as inhibit the drive enable/run commands.

This method has some risk in that if Bubba yanks open a disconnect while a drive is running, the drive enable may not drop out before the main poles open. Even so, this is not nearly as harmful to the drive as closing a disconnect with the drive already running. Hopefully, your folks aren't using the disconnect as a start/stop button so the drive will already be stopped when the motor is locked out.
 
One thing to keep in mind is that your motor isolator is probably rated for 60 Hz but with a VSD the frequency can get down to near DC which can have rather devastating effect on the isolator if the isolator is opened under load. The early break contacts would want to be early enough for the drive to die before the power contacts on the isolator open.
 
This method has some risk in that if Bubba yanks open a disconnect while a drive is running, the drive enable may not drop out before the main poles open. Even so, this is not nearly as harmful to the drive as closing a disconnect with the drive already running. Hopefully, your folks aren't using the disconnect as a start/stop button so the drive will already be stopped when the motor is locked out.

I should clarify that I wouldn't recommend this if avoidable, in my current situation, disconnects are locked out when the drive is already stopped or not enabled 99% of the time, and we run a contact directly to the drive programmed enable input (coast stop) but, probably 25% have a rusty wire-nut where the monitoring contact used to be, and are locked out frequently and by the same pretty well trained people. Our drives also have tight fusing upstream (I'd prefer a manual MCPB but, I digress.), speaking of PF70 and 700 with upstream line reactors and dual element fuses at the low end of the book recommendation.

Yes, hardwired enabled coast is going to be required for best results, and it would be wise to integrate the disconnect early break contact into any other safety related controls (like the reset of the power safety circuit if one exists).
 
Sorry for the delay guys. After sleeping on this I wonder if running the break before break contacts would be better as PLC inputs. These drives are about 100' from the drive but only about 15' from a RIO panel. The so it would be a nice and easy install. The customer wants 120VAC controls everywhere it is possible. The drives are 24VDC so I would have run 2 pipes to ever disconnect 480 and a 24. That could be a PITA forthwith electricians.
The only downside I see to this is the reaction time from the handle is pulled to the time the enable and run are stopped. The RPI can be 10 ms so that plus the scantime could be 20ms. Probably much less a 1768-L43S. We did a job like this a year ago with a MLX 1400 and external safety controls so the program isn't that bad.
 
I normally run a 'control' isolator into the PLC and just turn the enable output off.
For an emergency stop I normally run the contact from the emergency stop into thge drive and shut it down poste haste!
 
Jeff

The way I do this and IMO best practice is when the equipment needs to be put into a safe stopped state then use the normal stop command to stop the drive then when it is stopped remove the enable then timer then safe off bit.

One these are done on many of my lines then you can open the gate. I would not trust the timing of early break contacts to remove drive enable or safe stop. It is very possible you IGBT's will still be conducting when you break the phase contacts and spike the drive IGBT's and here comes smoke. This may work ok at first but as the mechanical springs of early break contacs wear not so much.

I you lose your safe off first you will coast stop and that may be ok or not depending on you app. If you don't want coast stop you will have to stop by stop command or drive enable loss and do a ramp depending on the drive model.

I normally use the disconnect aux for 2 things.

1. To let my plc / HMI that the disconnect is open and you can't start the motor until it is closed.

2. If the drive Aux is opened before the motor is stopped this is a estop condition and locks out the machine and can not be reset without a call to maintenance. A big horn goes off as well until you get that maintenance reset. This is also logged as a alarm. This is the only way we could prevent operators from blowing up drives here.
 

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